1. Field of the Invention
The present invention relates to a high-frequency filter, and more particularly, to a surface acoustic wave filter used in the high-frequency region.
2. Related Art of the Invention
In recent years, research has been earnestly carried out on surface acoustic wave devices for use as filters. Because of the recent development of mobile communication and the increase in frequency used for mobile communication, the development of surface acoustic wave devices, particularly, surface acoustic wave filters are actively worked on.
Conventionally, several methods have been known for forming a filter of the surface acoustic wave device in the high-frequency band, particularly, at several hundreds of MHz. Representative examples include one called a ladder type in which a filter is formed by use of a plurality of surface acoustic wave resonators as shown in Japanese Laid-open Patent Application S52-19044, one called a multi-electrode type as shown in Japanese Laid-open Patent Application S58-154917, a so-called longitudinal mode type in which surface acoustic wave resonators are adjacently disposed and the coupling between these resonators are used as shown in Japanese Laid-open Patent Applications Nos. H3-222512, S61-230419 and H1-231417.
Recently, with the development of mobile communication, the frequency band being used has been shifting to the quasi-microwave region. Previously, the characteristics of the surface acoustic wave filter depended on the electrode structure and the characteristics of the piezoelectric substrate. However, with the increase in frequency being used, other factors, particularly the influence of the parasitic capacitance and inductor cannot be disregarded at present. That is, because of the influence of the parasitic capacitance and inductor, it frequently occurs that the characteristics of the surface acoustic wave filter are different from expected characteristics.
For example, FIG. 24 shows calculated values of frequency characteristics of a three-electrode longitudinal-mode-type filter with a center frequency of 950 MHz using a LiNbO.sub.3 board of 64-degree Y cut and X propagation as the piezoelectric substrate. An example in which this type of filter is formed according to a conventional structure is shown in FIG. 26. That is, FIG. 26 shows the structure of a conventional high-frequency surface acoustic wave filter in which as disclosed in Japanese Laid-open Patent Application No. H4-33405, in order to obtain a ground terminal being excellent even at high frequencies, two ground terminals are connected for commonality on a piezoelectric substrate 101 as shown at 104a and the common ground terminal is electrically connected to a single terminal 104c for commonality in a package. Since like elements are designated like reference numerals, the description of FIG. 1 applies. Measured characteristics of the filter of FIG. 26 are as shown in FIG. 25. As is apparent from FIG. 25, in the conventional structure, the out-of-band attenuation amount particularly disagrees with the calculated values of FIG. 24.
Moreover, even in filters for the same system, desired filter characteristics vary among users. If this demand is met with the conventional filter structure, the design becomes complicated because of the presence of complicated inductors. This renders the control of production and stock cumbersome. A cause of the complicated inductors is that the ground terminals 104a and 104b are connected for commonality on the piezoelectric substrate 101 and the ground terminal 104c is connected for commonality in the package 110 as shown in FIG. 26 (see the view taken on the line B-B').
FIG. 28 shows the conventional high-frequency surface acoustic wave filter. The ground terminals are common in the package and there are parasitic capacitances existing in the high-frequency surface acoustic wave filter.